It is known in the prior art to use an electrolytic capacitor with a power supply line to suppress transient energy voltages by storing the energy within the capacitor, with the standard practice being to limit the so applied voltage to the surge voltage rating of the capacitor as specified by the capacitor manufacturer.
It is also known to suppress transient energy voltages by the use of a copper oxide rectifier above its voltage blocking limit so that transient energy is conducted through the device in the reverse direction to dissipate the transient in the form of heat. The copper oxide rectifier uses an oxide layer, which conducts in one direction and blocks in the other direction up to the breakdown voltage capability of the oxide layer, whereon the oxide layer behaves like a zener diode and conducts current above that breakdown voltage.
It is known in the prior art to employ thyristor switch devices in a chopper apparatus to control the current supplied to transient vehicle propulsion motors, as described in an article entitled, "Alternative Systems For Rapid Transit Propulsion And Electrical Braking" that was published by B. J. Krings at pages 34 to 41 of the Westinghouse Engineer for March 1973.